A polygon mirror arranged to receive a light flux emitted from a light source and configured to reflect and deflect the light flux is known in the art. Such a polygon mirror is subject to change in shape between a non-operating state and a stably-operating state which is brought about after operation for a predetermined period of time and in which the shape has been stabilized, due to centrifugal force produced by its spinning motion. Defocusing due to such change in shape may be corrected by providing a surface that is slightly recessed in the non-operating state.
In general, determination of design specifications of the light deflector is based, for convenience, on the premise that the reflecting surfaces of the polygon mirror are flat; thus, in order to obtain performance according to the required design specifications, it is preferable that the reflecting surfaces of the polygon mirror under operating conditions for deflecting a light flux be as nearly flat as possible.